A device-level identifier either IDFA (iOS) or GAID (Android) used to track ad exposure and attribute conversions in mobile environments.
MAID is the umbrella term for whichever mobile identifier a given OS uses IDFA on iOS, GAID on Android and it’s the identifier most mobile ad attribution has historically been built on, even as both platforms have moved toward requiring explicit user opt-in.
Further reading: Identity layer · Identity after third-party cookies
